In existing devices, to carry the film in use, two reels, a supply reel and a take up reel were employed. The film was wound off from the supply reel, passed through the projector system and wound up on the take up reel or rewound. As during such unreeling, the diameter of the film roll on the supply reel continuously decreased and on the take up reel continuously increased, the driving of the reels and thereby driving of the film strip through the shafts of the reels would have created a continuous change in the speed of the film. For that reason, in projectors or editors, the film had to be driven by a sprocket, the teeth of which fitted into the perforation of the film. The sprocket was driven by an independent power source, usually a reversible electric motor. The reels were driven independently from each other mostly through a spring belt and a clutch to create some slipping in the film to avoid too much tension.
In such devices, the film was dragged from the supply reel by a sprocket, through the perforations. A disadvantage of such prior system was that the film became easily damaged at the perforations and that the perforation in the film enlarged necessarily the width of the film by approximately 1/3 of its size. Besides this disadvantage, the prior art devices experienced difficulties especially in projectors with interrupted motion of the film, where the film strip passing through the projector window was driven with a jerking motion but the part of the film which passed through the sound reproducing system had to be driven with a steady flowing motion.
To solve this problem, three sprockets had to be used from which two sprockets played the role of a buffer to separate the jerking motion from the steady flowing part of the film.
The introduction of the optical compensators (prisms, etc.), in the field of projectors, however, opened the door to the use of a different driving system, making possible the elimination of the driving of the film through the perforation-sprocket system.